Caudal expiratory neurones in the rat

Abstract

The main source of expiratory drive to respiratory muscles in the rat is thought to be the caudal expiratory neurones. However, their projections to the spinal cord and the coordination of the population activity with the respiratory cycle are largely unknown. We examined their bulbospinal projections using antidromic activation, and the coordination of their activity using cross-correlation. Of 76 expiratory neurones examined, 40% projected to the C2 segment of the spinal cord, all unilaterally and all but 4 to the contralateral side. Thirteen contralateral axons were located in the ventromedial funiculus, when activation thresholds were less than 25 microA. For 29 neurones, the mean (+/- SEM) conduction velocity, calculated from the single-point activation and a measure of the direct distance between recording and stimulating electrodes, was 7.4 +/- 0.4 m/s. We calculated 88 cross-correlograms from ipsilateral pairs of expiratory neurones recorded on the same side of the medulla, and 176 contralateral pairs recorded from opposite sides of the medulla. All of the latter were featureless, but 23% of cross-correlograms for ipsilateral pairs displayed broad peaks at time zero, which we interpreted as due to activation of both neurones from a common source. We conclude that in the adult rat, approximately half of the caudal expiratory neurones project unilaterally and contralaterally to the spinal cord and, although common activation serves to coordinate some of the ipsilateral population, we suggest that neither common activation nor excitatory cross-connections exist as sufficient means for coordinating left and right populations to the same respiratory rhythm.

Respiratory Rhythm Entrainment by Somatic Afferent Stimulation

“…8 A, C,D). This is supported by previously published data Segers et al, 1987;Ezure, 1990;Bianchi et al, 1995;Okazaki et al, 2002;Shen and Duffin, 2002). As a result of frequency adaptation, the activity of post-I neurons is progressively reduced throughout expiration, thus decreasing post-I inhibition of the E2 neural population with time.…”

Inhibitory connections among rostral medullary expiratory neurones detected with cross-correlation in the decerebrate rat*

“…This percentage is low compared with the 23% for ipsilateral pairs of caudal VRG expiratory neurones (Shen and Duffin 2002) and 88% for ipsilateral pairs of inspiratory VRG neurones (Tian and Duffin 1997). The higher percentage of common activation for these latter neurones is consistent with their role in transmitting respiratory drive; such drives would be widely distributed among these populations.…”

“…Further support for direct connections to phrenic motoneurones comes from other studies demonstrating bulbospinal projections for E-DEC neurones using both antidromic activation (Shen and Duffin 2002) and anatomical labelling Zheng et al 1991a). However, the influence of E-DEC neurone inhibition may be weak since our attempts to detect an early expiratory inhibition of phrenic motoneurones in an Fig.…”

“…Those in the caudal part of the VRG that project to the spinal cord (Shen and Duffin 2002) are likely to control expiratory motoneurones (Iscoe 1998). In cats they project to the spinal cord (Arita et al 1987) and not to other parts of the medulla (Merrill 1974).…”

Whole Cell Recordings From Respiratory Neurones in an Arterially Perfused in situ Neonatal Rat Preparation

“…Lumsden, 1923;Cohen 1968Cohen , 1971von Euler, 1977;Berger et al 1978;St-John & Zhou, 1991;Dick et al 1994;Morrison et al 1994;Fung & St-John 1994, 1995. The majority of in vivo studies have focused on the ventrolateral regions of the medulla oblongata in adult mammals, where many rhythmic respiratory neurones reside in dog (Koepchen et al 1975;Stuth et al 1994), cat (Richter et al 1975(Richter et al , 1979Richter, 1982;Ezure 1990;Haji et al 2002), rat (Schwarzacher et al 1991;Duffin et al 2000;Dutschmann & Paton, 2002a;Ezure et al 2002;Shen & Duffin, 2002) and mouse (Paton, 1996a). However, the rat has become the species of choice for analysing the brainstem organization of both rhythm and pattern of respiration (Bianchi et al 1995).…”